1. Field of the Invention
The present invention relates to a non-reducible dielectric ceramic composition and, more particularly, to a non-reducible dielectric ceramic composition used for monolithic ceramic capacitors.
2. Description of the Prior Art
Monolithic ceramic capacitors are generally produced by first preparing dielectric ceramic green sheets with a dielectric ceramic composition such as a composition mainly comprising barium titanate, applying a conductive material for an internal electrode on one surface of each green sheet, stacking the green sheets, bonding them by thermocompression, cutting the resultant multilayered body into ceramic capacitor green chips, then firing the green chips in air at 1250.degree.-1350.degree. C. to form capacitor chips with internal electrodes, and finally forming external electrodes connected to the internal electrodes on opposed sides of each of the capacitor chips to complete monolithic ceramic capacitors.
Accordingly, a conductive material for internal electrodes is required to meet conditions such that (a) it has a melting point higher than a sintering temperature of the dielectric ceramics as the internal electrodes are fired along with the dielectric ceramic material; and that (b) it neither oxidize nor react with the dielectric ceramics even in an oxidizing atmosphere with a high temperature.
To meet such requirements, noble metals such as platinum, gold, palladium and their alloys have been used as a conductive material for the internal electrodes. Such noble metals provide satisfactory characteristics, but for all practical purposes they are too expensive to use. That is, use of such a noble metal is the greatest factor determining the rise in the production cost of monolithic ceramic capacitors as the cost of internal electrodes occupies from 30 to 70% of the production cost of monolithic ceramic capacitors.
Other metals having a high melting point are base metals such as Ni, Fe, Co, W and Mo, but they are easily oxidized in a oxidizing high temperature atmosphere. Thus, if such a base metal is used as a material for internal electrodes of monolithic ceramic capacitors, it loses the function serving as the internal electrodes when the green chips are fired in air. It is therefore required to fire the green chips in a neutral or reducing atmosphere in order to use a base metal as a material for internal electrodes of monolithic ceramic capacitors. However, the dielectric ceramic materials of the prior art are reduced considerably and thus semiconductorized when fired in the reducing atmosphere.
To solve such a problem, it has been proposed in JP-B-57-42588 to use a dielectric ceramic material comprising a barium titanate solid solution having a ratio of a barium site to a titanium site larger than the stoichiometric ratio. Such a dielectric ceramic material is scarcely semiconductorized even if fired in a reducing atmosphere, thus making it possible to manufacture monolithic ceramic capacitors including internal electrodes of a base metal such as nickel. It is, however, difficult with such dielectric ceramic materials to miniaturize monolithic ceramic capacitors for the following reasons.
With recent development of electronic techniques, considerable progress has made in miniaturization of electronic devices. Consequently, this requires miniaturization of monolithic ceramic capacitors as well as other electronic parts. For the monolithic ceramic capacitors, the miniaturization is generally carried out by using a dielectric ceramic material with a high dielectric constant or by making dielectric ceramic layers thin. The dielectric ceramic materials as disclosed in JP-B-57-42588 have high dielectric constant, but they are large in grain size. Thus, if the thickness of dielectric ceramic layers is decreased to 10 .mu.m or below, the number of crystal grains present in each layer is considerably decreased, resulting in lowering of reliability of the monolithic ceramic capacitors.
On the other hand, JP-B-61-101459 discloses a nonreducible dielectric ceramic composition consisting of a barium titanate solid solution into which a rare earth element such as La, Nd, Sm and Dy has been incorporated. Such a composition has a fine grain size which enables one to avoid lowering of the reliability of monolithic ceramic capacitors as the fine grains size contributes to increase the number of crystal grains present in each dielectric layer. However, it is impossible with such a ceramic composition to obtain high dielectric constant and that the composition containing a rare earth element is easy to oxidize during firing.